Methodology and sources
Where UnitGlide’s numbers come from, and why they match the answer you would get working the math out by hand.
Every conversion on UnitGlide is computed from a single, auditable set of factors. Within a category, each unit stores how many base units it equals, and a conversion is just value × from-factor ÷ to-factor. Temperature is the one exception: because each scale has a different zero point, those conversions use exact formulas through kelvin rather than a multiplier. No result is a figure copied off a chart, and none is rounded before it reaches you: the factors are stored at full precision and only the displayed value is trimmed for readability.
The factors themselves are the ones defined by the international measurement authorities: the BIPM for the SI, and NIST for the US customary and legacy units. Where a unit is exact by definition (an inch is exactly 25.4 mm, a pound is exactly 0.45359237 kg), we keep it exact.
How each category is anchored
Length base unit: metre (m)
The metre is an SI base unit, fixed by the defined value of the speed of light. Imperial lengths are exact multiples set by the 1959 international yard and pound agreement: 1 inch = 25.4 mm, 1 foot = 0.3048 m, 1 yard = 0.9144 m and 1 mile = 1609.344 m. The nautical mile is exactly 1852 m.
Sources
Mass base unit: kilogram (kg)
The kilogram is an SI base unit, defined since 2019 by the fixed value of the Planck constant. The avoirdupois pound is exactly 0.45359237 kg; one ounce is one sixteenth of that (28.349523125 g) and one stone is 14 pounds (6.35029318 kg).
Sources
Temperature base unit: kelvin (K)
Temperature scales share no common zero, so they cannot be converted with a single factor. UnitGlide converts through kelvin using the exact scale formulas: °F = (°C × 9/5) + 32, with water freezing at 0 °C / 32 °F and boiling at 100 °C / 212 °F. The kelvin is fixed by the defined value of the Boltzmann constant.
Sources
Volume base unit: litre (L)
One litre is exactly one cubic decimetre (0.001 m³). UnitGlide uses US customary fluid measures: the US gallon is exactly 231 cubic inches (3.785411784 L), the US fluid ounce is 1/128 of that (29.5735295625 mL). Imperial (UK) equivalents (gallon 4.54609 L, fluid ounce 28.4130625 mL) are stated on the relevant pages.
Sources
Area base unit: square metre (m²)
Areas are anchored to the square metre. Land units are exact: one acre is 4046.8564224 m² and one hectare is exactly 10 000 m². Square imperial units follow from the exact linear factors (1 ft² = 0.09290304 m²).
Sources
Speed base unit: metre per second (m/s)
Speeds are referenced to the metre per second. One kilometre per hour is exactly 1/3.6 m/s, one mile per hour is exactly 0.44704 m/s, and one knot is one nautical mile per hour (1852 m/h).
Sources
Time base unit: second (s)
The second is an SI base unit, defined by the caesium-133 hyperfine transition frequency. Larger units are exact multiples (minute 60 s, hour 3600 s, day 86 400 s). The year here is the Julian year of 365.25 days used in astronomy.
Sources
Digital storage base unit: byte (B)
UnitGlide uses the decimal (SI) convention that drive makers and networks use: 1 kB = 1000 bytes, 1 MB = 10⁶ bytes, and so on. The binary sizes (KiB, MiB, GiB) that some operating systems label as KB/MB/GB are a separate system defined by IEC 80000-13.
Sources
Pressure base unit: pascal (Pa)
Pressures are anchored to the pascal. Common units are exact or standardised multiples: 1 bar = 100 000 Pa, 1 standard atmosphere = 101 325 Pa, and 1 psi = 6894.757293168 Pa.
Sources
Energy base unit: joule (J)
Energies are referenced to the joule. The thermochemical calorie is exactly 4.184 J, the (food) kilocalorie is 4184 J, one watt-hour is 3600 J and one BTU is 1055.05585262 J.
Sources
Power base unit: watt (W)
Powers are anchored to the watt. Mechanical horsepower is 745.6998715822702 W and metric horsepower (PS) is exactly 735.49875 W.
Sources
Typography base unit: pixel (px)
Typography units use the fixed CSS reference where 1 inch = 96 px = 72 pt. One rem equals the common 16 px default browser root font-size; scale accordingly if a project sets a different root size. These are CSS specification values, not SI figures.
Sources
Precision and rounding
Factors are stored to their full defined precision, so intermediate math carries no rounding error. The converter trims the displayedanswer to a sensible number of significant figures, more for small values and fewer for large ones, and removes trailing zeros. If you need every digit, the underlying factor for any pair is shown in the “where the factor comes from” section on that pair’s page.
Primary references
- BIPM, The International System of Units (SI Brochure, 9th ed.)
- NIST Special Publication 811, Guide for the Use of the International System of Units
- NIST Handbook 44, Appendix C: General Tables of Units of Measurement
- NIST, Kelvin: Introduction
- IEC 80000-13, Quantities and units — Information science and technology
- W3C, CSS Values and Units Module (absolute lengths)
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